The multigenic immunoglobulin heavy and light chain variable (VH and VL) region families consist of chromosomally separated genetic elements which undergo unique rearrangements and somatic variation during lymphoid differentiation. Recombinant DNA approaches will be used to characterize V genes in species representative of the earliest stages of vertebrate phylogenetic development. These include Petromyzon (sea lamprey), a modern representative of the ancient jawless vertebrates, which elicits specific immune responses but lacks a spleen or thymus, and Heterodontus (horned shark), an Elasmobranch, which possesses both a spleen and a thymus, exhibits a complex antibody repertoire but synthesizes only a single, IgM-like immunoglobulin class. Affinity maturation, characteristic of higher vertebrate antibody, is absent in Heterodontus. VH, Vl and constant region sequences will be identified in cosmid- or Lambda phage-Heterodontus genomic DNA libraries using synthetic oligodeoxynucleotide probes. Gene numbers and linkage relationships will be examined and somatic variation will be studied in cDNA libraries constructed with mRNAs isolated from the lymphoid tissue of vaccine hyperimmunized individuals. Oligonucleotide probing and mRNA enrichment techniques will be used to identify genes associated with the immune response of Petromyzon. The relationship of the phylogenetically ubiquitous heavy chain switch sequence to the immunoglobulin gene system in both species will be studied by restriction mapping. Metric analysis will facilitate analysis of sequence data and identification of sequence features which are associated with specific functions and reflect patterns of evolutionary development. Taken together, the studies will define fundamental genetic mechanisms at a point in evolutionary time prior to acquisition of the complexity evident in the immune system of modern mammals. At earlier stages of phylogenetic development it is possible that elements of the immunoglobulin gene reorganization mechanism may be components of other developmental processes unrelated to antibody production. The known relationship between components of the segmental rearrangement process and certain lymphoid malignancies suggests that insight into the relationship between normal and neoplastic growth, such as in the human cancers, will be gained.